The importance of high density lipoproteins (HDL) in protecting against the development of atherosclerosis has been well established. While the mechanism by which HDL protects against atheroma formation is presumed to be secondary to it' s role in reverse cholesterol transport, the structural requirements for that function are uncertain. Genetic disorders resulting in abnormal HDL particles may shed light on this question. We have identified a 48 year old woman with HDL cholesterol levels in the 0-13 mg/dl range. Using the polymerase chain reaction, we have tentatively identified a mutation in her apo A-I gene. Surprisingly, despite her extremely low HDL cholesterol levels, she has no evidence of CAD. The exploration of the relevance of this putative mutation to HDL function may illuminate aspects of the A-I apoprotein's structure/function relationships, and aid in the understanding of the protective effect of HDL against coronary artery disease. We plan to investigate HDL particle structure in the proband by immunoaffinity column chromatography, SDS polyacrylamide gel electrophoresis, and non-denaturing gradient polyacrylamide gel electrophoresis. Apo A-I structure will be assessed by two dimensional gel electrophoresis. The apo A-I gene will be assessed with techniques including the polymerase chain reaction, Southern blotting, creating and screening a genomic DNA library, and DNA sequencing. Metabolism of apo A-I will be studied by an endogenous labeling protocol with trideuterated leucine as the tracer.